Add a CreateCodeRefactoringContext overload

This allows the derived unit test to provide custom internal options,
similar to what we do in 275a14bd98.

src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.CodeRefactoring.Testing/CodeRefactoringTest`1.cs

@@ -65,6 +65,17 @@ namespace Microsoft.CodeAnalysis.Testing
         /// <returns>The <see cref="CodeRefactoringProvider"/> to be used.</returns>
         protected abstract IEnumerable<CodeRefactoringProvider> GetCodeRefactoringProviders();
 
+        /// <summary>
+        /// Creates a code refactoring context to be used for testing.
+        /// </summary>
+        /// <param name="document">Document to refactor.</param>
+        /// <param name="span">Text span within the <paramref name="document"/> to refactor.</param>
+        /// <param name="registerRefactoring">Delegate to register a <see cref="CodeAction"/> for the refactoring.</param>
+        /// <param name="cancellationToken">Cancellation token.</param>
+        /// <returns>New <see cref="CodeRefactoringContext"/>.</returns>
+        protected virtual CodeRefactoringContext CreateCodeRefactoringContext(Document document, TextSpan span, Action<CodeAction> registerRefactoring, CancellationToken cancellationToken)
+            => new CodeRefactoringContext(document, span, registerRefactoring, cancellationToken);
+
         protected override async Task RunImplAsync(CancellationToken cancellationToken)
         {
             Verify.NotEmpty($"{nameof(TestState)}.{nameof(SolutionState.Sources)}", TestState.Sources);
@@ -258,7 +269,7 @@ namespace Microsoft.CodeAnalysis.Testing
 
                 foreach (var codeRefactoringProvider in codeRefactoringProviders)
                 {
-                    var context = new CodeRefactoringContext(triggerDocument, location.SourceSpan, actions.Add, cancellationToken);
+                    var context = CreateCodeRefactoringContext(triggerDocument, location.SourceSpan, actions.Add, cancellationToken);
                     await codeRefactoringProvider.ComputeRefactoringsAsync(context).ConfigureAwait(false);
                 }
 

src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.CodeRefactoring.Testing/PublicAPI.Unshipped.txt

@@ -18,3 +18,4 @@ static Microsoft.CodeAnalysis.Testing.CodeRefactoringTest<TVerifier>.TriggerSpan
 static Microsoft.CodeAnalysis.Testing.CodeRefactoringVerifier<TCodeRefactoring, TTest, TVerifier>.VerifyRefactoringAsync(string source, Microsoft.CodeAnalysis.Testing.DiagnosticResult expected, string fixedSource) -> System.Threading.Tasks.Task
 static Microsoft.CodeAnalysis.Testing.CodeRefactoringVerifier<TCodeRefactoring, TTest, TVerifier>.VerifyRefactoringAsync(string source, Microsoft.CodeAnalysis.Testing.DiagnosticResult[] expected, string fixedSource) -> System.Threading.Tasks.Task
 static Microsoft.CodeAnalysis.Testing.CodeRefactoringVerifier<TCodeRefactoring, TTest, TVerifier>.VerifyRefactoringAsync(string source, string fixedSource) -> System.Threading.Tasks.Task
+virtual Microsoft.CodeAnalysis.Testing.CodeRefactoringTest<TVerifier>.CreateCodeRefactoringContext(Microsoft.CodeAnalysis.Document document, Microsoft.CodeAnalysis.Text.TextSpan span, System.Action<Microsoft.CodeAnalysis.CodeActions.CodeAction> registerRefactoring, System.Threading.CancellationToken cancellationToken) -> Microsoft.CodeAnalysis.CodeRefactorings.CodeRefactoringContext